Method of forming a semiconductor device and semiconductor device

ABSTRACT

A method of forming a semiconductor device comprises forming a control electrode over a portion of a semiconductor layer, forming recesses extending into the semiconductor layer on opposing sides of the control electrode, and forming doped regions in the semiconductor layer through the recesses. The doped regions form current electrode regions of the semiconductor device and each doped region extends into the semiconductor layer from at least a base of a recess. The method further comprises forming, after forming the doped regions, strained semiconductor regions in the recesses, wherein a junction between each doped region and the semiconductor layer is formed below an interface between a strained semiconductor region and the semiconductor layer.

FIELD OF THE INVENTION

This invention relates to a method of forming a semiconductor device anda semiconductor device. More particularly, this invention relates toimproving semiconductor device performance with the use of strainengineering.

BACKGROUND OF THE INVENTION

Advances in semiconductor device fabrication processes have led toscaling of device dimensions which has resulted in improvements indevice performance and cost. For CMOS transistor devices, aftersignificant scaling over the years, due to limitations in gate-oxidethickness and source/drain (S/D) junction depth, it appears that thescaling limit is being approached, where more shrinking will not boostperformance because of leakage, power dissipation and tunnellingeffects. Thus, techniques, other than scaling, are being considered inorder to maintain and/or improve device performance for smaller devicesizes. Strain engineering is one such technique.

Strain engineering involves straining of the semiconductor crystal toincrease the mobility of charge carriers in the channel (electrons inN-channel MOSFET (NMOS) devices and holes in P-channel MOSFET (PMOS)devices). Device performance may be improved with enhanced carriermobility. Compressive strain is induced in silicon-based PMOS devices,typically using epitaxially grown silicon and germanium SiGe in thesource and drain regions of the device. This is known as a source/drainstressor structure. In silicon-based NMOS devices, a tensile strain isused. By adding stress to a channel of a transistor, the carriermobility in the channel is enhanced which increases the drive current ofthe transistor but without impacting negatively other device parameterssuch as off-state leakage.

In a conventional process flow to fabricate a source/drain stressorstructure in a transistor device, as shown for example in U.S. Pat. No.6,861,318, the source/drain recesses are etched in a silicon substrateto the desired depth of the stressor structure and material, typicallySiGe, is epitaxially deposited in the etched recesses. The epitaxiallydeposited material may be doped in-situ (in other words, the strainedmaterial includes a dopant in addition to silicon and germanium), whilein other cases the deposited SiGe is implanted with dopants which arethen activated using a thermal anneal process.

A disadvantage to using the SiGe S/D stressor structure for generalpurpose or low power applications (for example, in battery operatedapplications such as mobile devices) is that the SiGe as a material hasinherently higher junction leakage than silicon and in many casesdefects are formed at the interface between the SiGe and the siliconsubstrate that also can form leakage paths through the junction. Thismay result in power being wasted due to junction leakage.

There is therefore a need to provide a semiconductor device which usesstrain engineering but which attempts to reduce junction leakage.

SUMMARY OF THE INVENTION

The present invention provides a method of forming a semiconductordevice and a semiconductor device as described in the accompanyingclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

A semiconductor device and a method of forming a semiconductor device inaccordance with the present invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIGS. 1-4 are cross-sectional side views of part of a semiconductordevice during different stages of manufacture;

FIG. 5 is a graph showing the relationship between the depth of thestrained semiconductor region in a semiconductor device and the leakagecurrent;

FIGS. 6-9 are cross-sectional side views of part of a semiconductordevice during different stages of manufacture in accordance with oneembodiment of the invention; and

FIGS. 10-12 are cross-sectional side views of part of a semiconductordevice during different stages of manufacture in accordance with anotherembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the description that follows and in FIGS. 1-12, certainregions/layers are identified as being of a particular material,conductivity and/or type. However, this is merely for convenience ofexplanation and not intended to be limiting. Those of skill in the artwill understand based on the description given herein that varioussemiconductor materials can be used and that the doping of variousregions of the device may be altered in order to obtain different devicefunctions. Note, the Figures are not drawn to scale.

The present invention will be described with reference to asemiconductor device comprising a planar silicon-based PMOS device. Itwill be appreciated that the invention is not limited to a planarsilicon-based PMOS device and applies equally to other semiconductordevices, such as NMOS device, III-V semiconductor-based device, ornon-planar device such as a FinFET device or similar devices.

A known process for forming a PMOS device having a S/D stressorstructure will now be described with reference to FIGS. 1-4.

FIG. 1 shows a partially manufactured PMOS device on an integratedcircuit comprising an epitaxial silicon layer 4 formed on a substrate 2,isolation regions 6 (e.g. oxide isolation regions) formed in theepitaxial silicon layer 4 to isolate wells of different conductivitytypes and to isolate adjacent transistors and a gate electrode 8 formedon a gate dielectric layer 10. The gate electrode may be a polysiliconelectrode and the gate dielectric layer 10 may be a nitrided oxidelayer. Lightly doped or doped shallow regions 12 are formed in theepitaxial silicon layer 4 and are located on opposing sides of the gateelectrode 8. These regions 12 are also known as lightly doped drainregions (in fact a region 12 on one side of the gate electrode 8 is alightly doped source region and a region 12 on the other side is alightly doped drain region) or extension regions. For PMOS device, thedopant for regions 12 may be boron. Spacers 14 are formed on opposingsides of the gate electrode 8. The spacers 14 cover the sides of thegate electrode 8 and portions of a surface 16 of the epitaxial siliconlayer 4. The spacers 14 may be nitride spacers. A hard mask 18, such asa 20 nm oxide mask formed by PECVD, is then formed over thesemiconductor device and photoresist 20 is then formed over the othernon-PMOS devices on the integrated circuit.

FIG. 2 shows the structure of FIG. 1 after a selective etch step. Ananisotropic etchant is used which selectively removes the hard mask 18and the epitaxial silicon layer 4 which are not covered with photoresist20 to form recesses 22 on opposing sides of the gate electrode 8. Thegate electrode 8 is also etched. In other process flows, a mask is usedover the gate such that the gate electrode 8 is not etched. The recessesextend about 65 nm from the surface 16 into the epitaxial silicon layer4. Inner edges of the recesses undercut the outer edges of the spacers14 and outer edges of the recesses are aligned with the isolationregions 6. The exposed portion of lightly doped drain regions 12 arealso etched in the selective etch step to provide lightly doped drain(and source) regions 13 and areas for deposition of the stressormaterial, which will be doped to provide the deep source drain regionsof the device.

As shown in FIG. 3, the photoresist 20 is then removed and asemiconductor material 24 is formed in the recesses 22 such that itfills the recesses 22 and extends above the surface 16 of the epitaxialsilicon layer 4. The semiconductor material 24 in the recesses is chosensuch that it forms strained semiconductor regions 24 in the epitaxialsilicon layer 4. The semiconductor material 24 thus preferably has alattice spacing greater than the lattice spacing of the epitaxialsilicon layer 4. The semiconductor material typically includesepitaxially grown silicon and germanium and boron and is deposited onthe epitaxial silicon layer 4 in the recesses 22. If the polysilicongate electrode 8 is etched, then the strained semiconductor material isalso deposited on the polysilicon gate electrode 8.

FIG. 4 shows the formation of the drain and source regions. Aphotoresist 26 is formed over the integrated circuit leaving openings 25for the PMOS devices. A dopant, such as boron, is then implanted throughthe strained semiconductor regions 24 in the recesses 22 into theepitaxial silicon layer 24 to form source and drain regions 28.

In use of the PMOS device with a S/D stressor structure, a voltage canbe applied over the source and drain regions 28. Current flows from thesource region to the drain region through a channel 30 below the gatedielectric layer 10 when a voltage is applied to the gate electrode 8.As discussed above, the lattice of the strained semiconductor regions24, comprising silicon and germanium, has a larger spacing than thespacing of the lattice of the epitaxial silicon layer 4 and because ofthis larger spacing, the strained semiconductor regions 24 create acompressive stress in the channel 30 which changes the mobility of thecarriers in the channel. As discussed above, this has the advantageouseffect of increasing the drive current.

The inventors of the subject application have investigated the effect ofthe structure of the S/D stressor structure on the junction leakage andhave discovered that the positioning of the junction between the sourceand drain regions and the epitaxial silicon layer with respect to theinterface between the strained semiconductor regions and the epitaxialsilicon layer has a significant impact on the junction leakage current.As is well known, the junction leakage is the leakage from the junctionsto a well.

FIG. 5 shows the junction leakage current Ibulk of a PMOS device with aS/D stressor structure for different depths of the recesses 22 (i.e.different depths of the strained SiGe regions 24 in the epitaxialsilicon layer 4) when the implant process parameters are kept the same.The reference corresponds to a device without the strained SiGe regions24. With depths 2 and 3 in FIG. 5, the source and drain junctions areformed in the strained SiGe regions and the junction leakage current issignificantly increased. With depth 1, the source and drain junctionsare formed in the epitaxial-silicon layer 4 below the strained SiGeregions and the leakage current is at the desired reference level.

Thus, from their investigations, the inventors have recognised that inorder to reduce leakage current and hence wasted power, the source anddrain regions should be formed below the strained regions.

Having deeper strained SiGe regions provide higher stress in the channeland so higher performance but too deep a strained region can createunacceptably high off-state leakage, which occurs only in the channel ofthe device between the source and drain and is due to the high fields inthe channel between the source and drain junctions, and dispersion.Also, as the depth of the strained SiGe regions increases so does thedepth of the implant required to form the source and drain regions belowthe strained SiGe regions since the dopants have to be implanted throughdeeper SiGe regions. Deeper implants cause the implanted junctions to bespread out. In order to maintain or improve device performance,shallower or more abrupt implanted junctions are desired.

A semiconductor device and a method of forming a semiconductor device inaccordance with the present invention will now be described withreference further to FIGS. 6-9. Like components to those of FIGS. 1-4are referenced by the same reference numeral plus the number 100.

FIG. 6 shows a partially manufactured PMOS device of an integratedcircuit (not shown) comprising a semiconductor layer 104, such as anepitaxial silicon layer or a silicon layer, formed on a substrate 102,such as a monocrystalline silicon substrate. The semiconductor layer 104may be doped to form wells as is well known in the art. Isolationregions 106 are formed in the epitaxial silicon layer 104 to isolatewells of different conductivity types and to isolate adjacenttransistors. Isolation regions 106 may be shallow trench isolationregions and may be formed of an oxide. A control electrode 108, whichfor a MOSFET device is the gate electrode, is formed over a portion ofthe epitaxial silicon layer 104 and formed on a gate dielectric layer110, such as a nitrided oxide layer. The gate electrode 108 may beformed by deposition of polysilicon using known techniques.

Lightly doped or doped shallow regions 112 are formed in the epitaxialsilicon layer 104 and are located on opposing sides of the gateelectrode 108. These regions 112 are also known as lightly doped drainregions (a region 112 on one side of the gate electrode 108 is a lightlydoped source region and a region 112 on the other side is a lightlydoped drain region) or extension regions. For a PMOS device, theextension regions 112 may be formed by implanting boron ions into asurface 116 of the epitaxial silicon layer 104. Spacers 114 are formedon opposing sides of the gate electrode 108. The spacers 114 cover thesides of the gate electrode 108 and portions of the surface 116 of theepitaxial silicon layer 104. The spacers 114 may be nitride spacers andmay be formed by well known techniques.

A hard mask 118, such as a 20 nm oxide mask, is formed, for example byPECVD, over the semiconductor device and photoresist 120 is then formedover the other non-PMOS devices on the integrated circuit.

Referring now to FIG. 7, a selective etch step is then performed inorder to form recesses 122 extending into the epitaxial silicon layer104 from the surface 116 on opposing sides of the gate electrode 108.These recesses 122 define the source/drain stressor regions. Ananisotropic etchant may be used which selectively removes the hard mask118 and the epitaxial silicon layer 104 which are not covered withphotoresist 120 to form recesses 122. In the process flow shown herein,the gate electrode 108 is also etched. In other process flows, however,a mask may be used such that the gate electrode may not be etched. Theprocess parameters for the selective etch step are chosen such that therecesses extend between 60-100 nm from the surface 116 into theepitaxial silicon layer 104. In the embodiment shown in FIG. 7, inneredges of the recesses 122 undercut the outer edges of the spacers 114and outer edges of the recesses 122 are aligned with the isolationregions 106. The exposed portions of the lightly doped drain regions 112are also etched in the selective etch step to provide lightly dopeddrain (and source) regions 113.

Doped regions 150 are then formed in the epitaxial silicon layer 104through the recesses 122 such that each of the doped regions 150 extendinto the epitaxial layer 104 from at least a base of the recess (seeFIG. 8). The doped regions 150 form the current electrode regions of thesemiconductor device: for MOSFET devices, these are the source and drainregions. The doped regions 150 may be formed by implanting a dopant,such as boron for PMOS devices, having a high dose, for example in therange of 1e15 cm⁻² to 1e17 cm⁻² and a low implant energy, for example,equal to or less than 1 keV. The implantation energy for implantingdoped regions 150 is increased compared to the implantation energy usedto form lightly doped regions 112 but is selected to be low so as toform shallow implanted regions 150, typically extending to a depth of 80nm from the surface 116: shallow implants give better control overjunction abruptness. The dopant ions may be implanted in a direction ofimplant at an angle 152 in the range of 0-15° to a normal to the surface116 of the epitaxial silicon layer 104. Implanting at a tilt angle is awell known technique. As is clear from simple geometry, implanting at anangle of say 10° means that the junction between a doped region 150 andthe epitaxial silicon layer 104 is further away from the vertical edgeof a recess which, as discussed above, when the recess is filled withsemiconductor material to become a strained region, helps reduce leakagecurrent.

Referring now to FIG. 9, the photoresist 120 is then removed and asemiconductor material 124 is formed in the recesses 122 such that itfills the recesses 122 and in the embodiment shown in FIG. 9, extendsabove the surface 116 of the epitaxial silicon layer 104. Thesemiconductor material 124 in the recesses is chosen such that it formsstrained semiconductor regions 124 in the epitaxial silicon layer 104.In an embodiment of the invention, the semiconductor material 124 has alattice structure that is substantially the same as the latticestructure of the semiconductor layer 104 and has a lattice spacing orlattice constant (which is the distance between atoms in the lattice)different to the lattice spacing of the epitaxial silicon layer 104. Thedifference in lattice spacing should be large enough to create strainbut not too large to create defects that relieve the strain. To induce acompressive strain for PMOS devices, the lattice spacing of thesemiconductor material 124 is required to be greater than the latticespacing of the semiconductor layer 104. Other factors may be consideredwhen selecting the semiconductor material 124, for example, it isdesirable that the semiconductor material 124 is compatible with thesemiconductor layer 104 upon which it is deposited at the temperaturesused in the semiconductor device fabrication process.

The semiconductor material 124 typically includes epitaxially grownsilicon and germanium and boron, known as in-situ boron doped SiGe, andis formed by epitaxial deposition on the epitaxial silicon layer 104 inthe recesses 122, for example, by a rapid thermal CVD process.Typically, the percentage of silicon used in the SiGe material 124 is25-30%. The percentage is determined such that the addition of Ge to theSi changes the lattice constant sufficiently to create strain withoutcreating defects that relieve the strain. When too much Ge is present,the lattice constant difference between the semiconductor layer 104 andthe stressor semiconductor material 124 becomes so large that defectsform to relieve the high stress induced by the lattice constantdifference. Thus, the percentage of Si vs Ge is determined by the tradeoff between wanting high stress in the channel but not so high thatdefects form to relieve the stress and create leakage paths through thejunction. Other semiconductor materials may also be used to fillrecesses 122, such as undoped silicon and germanium or othercombinations of III-IV semiconductor materials such as gallium andarsenide.

Instead of having an epitaxial silicon layer 104, the semiconductorlayer 104 may comprises a conventional prime substrate, such as asilicon substrate. In this case, the stressor semiconductor material 124may comprise silicon and germanium (or in-situ doped SiGe) epitaxiallydeposited on the silicon substrate.

If the polysilicon gate electrode 108 is etched, then the semiconductormaterial is also deposited on the polysilicon gate electrode 108.

The junction between the source and drain regions 150 and the epitaxialsilicon layer 104 is formed a predetermined depth 151 below theinterface between the strained semiconductor regions 124 and theepitaxial silicon layer 104. The predetermined depth is in the range of5-10 nm and is determined by the energy used to form the source anddrain regions 150 (i.e. the implant energy) and the contour of therecesses 122 through which the source and drain regions 150 are formed.

Other process steps then take, place to complete the formation of thePMOS device which are well known in the art, for example, thermalprocesses to activate the implanted materials, metallization to form thesource, drain and gate electrodes.

By forming the source and drain regions before forming the strainedsemiconductor regions, the present invention ensures that the junctionbetween the source and drain regions and the epitaxial silicon layer isbelow the interface between the strained semiconductor region and theepitaxial silicon layer. This has an advantage, as can be seen in FIG. 5above, of reducing junction leakage which is particularly advantageousin low power applications. Furthermore, by forming the source and drainregions before forming the strained semiconductor regions, the presentinvention allows for the source and drain regions to be formed by ashallow implant which gives better control over the junction abruptnessand hence improves device performance. Moreover, the present inventionenables the depth of the strained semiconductor regions to be increasedwithout requiring deeper implants in order to form the source and drainregions below the strained semiconductor regions. As discussed above,having deeper strained semiconductor regions increases the stress on thechannel which increases charge mobility. Thus, the present inventionenables the structure of a source/drain stressor PMOS device to beoptimised to improve device performance.

By comparing the known process flow described above with respect toFIGS. 1-4 and the process flow in accordance with the inventiondescribed above with respect to FIGS. 6-9, the present invention usesthe same photoresist 120 to form the recesses and the source and drainregions (see FIG. 8) whereas the prior art process flow requires anadditional photoresist 26 (see FIG. 4) in order to form the source anddrain regions. Thus, by avoiding the need for additional processingsteps (e.g. photolithography), the present invention reduces the cost ofmanufacture of such strained devices.

A method in accordance with a second aspect of the invention will now bedescribed with reference to FIGS. 10-12. Like components to those ofFIGS. 6-9 are referenced by the same reference numeral plus 100.

FIG. 10 shows a partially manufactured PMOS device of an integratedcircuit (not shown) comprising a semiconductor layer 204, such as anepitaxial silicon layer or silicon layer, formed on a substrate 202,such as a monocrystalline silicon substrate. The semiconductor layer 204may be doped to form wells as is well known in the art. Isolationregions 206 are formed in the epitaxial silicon layer 204 to isolatewells of different conductivity types and to isolate adjacenttransistors. Isolation regions 206 may be shallow trench isolationregions and may be formed of an oxide. A control electrode 208, whichfor a MOSFET device is the gate electrode, is formed over a portion ofthe epitaxial silicon layer 204 and formed on a gate dielectric layer210, such as a nitrided oxide layer. The gate electrode 208 may beformed by deposition of polysilicon using known techniques.

Lightly doped or doped shallow regions 212 are formed in the epitaxialsilicon layer 204 and are located on opposing sides of the gateelectrode 208. These regions 212 are also known as lightly doped drainregions (a region 212 on one side of the gate electrode 208 is a lightlydoped source region and a region 212 on the other side is a lightlydoped drain region) or extension regions. For a PMOS device, theextension regions 212 may be formed by implanting boron ions into asurface 216 of the epitaxial silicon layer 204. Spacers 214 are formedon opposing sides of the gate electrode 208. The spacers 214 cover thesides of the gate electrode 208 and portions of the surface 216 of theepitaxial silicon layer 204. The spacers 214 may be nitride spacers andmay be formed by well known techniques.

A hard mask 218, such as a 20 nm oxide mask, is formed, for example byPECVD, over the semiconductor device and photoresist 220 is then formedover the other non-PMOS devices on the integrated circuit.

Referring now to FIG. 11, a selective etch step is then performed inorder to form recesses 222 extending into the epitaxial silicon layer204 from the surface 216 on opposing sides of the gate electrode 208.These recesses 222 define the source/drain stressor regions. Ananisotropic etchant may be used which selectively removes the hard mask218 and the epitaxial silicon layer 204 which are not covered withphotoresist 220 to form recesses 222. In the process flow shown herein,the gate electrode 208 is also etched. In other process flows, however,a mask may be used such that the gate electrode may not be etched. Theprocess parameters for the selective etch step are chosen such that therecesses extend between 60-100 nm from the surface 216 into theepitaxial silicon layer 204. In the embodiment shown in FIG. 11, theinner edges of the recesses 222 undercut the outer edges of the spacers214 and outer edges of the recesses 222 are aligned with the isolationregions 206. The exposed portions of the lightly doped drain regions 212are also etched in the selective etch step to provide lightly dopeddrain (and source) regions 213.

Referring now to FIG. 12, the photoresist 220 is then removed and afirst layer 270 of a first semiconductor material doped with a dopant isformed in the recesses 222. The first semiconductor material typicallyincludes epitaxially grown silicon and boron, known as in-situ borondoped silicon and is formed by epitaxial deposition on the epitaxialsilicon layer 204 in the recesses 222 typically using a rapid thermalCVD process. Note, deposition also occurs on the side of the recesses222 that are adjacent to the channel but does not occur on the sides ofthe recess that are adjacent to the isolation regions 206. In anembodiment, the first layer 270 has a thickness of 2-5 nm. The dopedfirst layer 270 in each recess 222 forms a current electrode region 270(e.g. source or drain region) of the device. Other semiconductormaterials and dopants may be used in the first layer 270 to form thesource and drain regions 270. For example, SiC for NMOS devices.

If the gate electrode 208 is etched, then the first semiconductormaterial is also deposited on the polysilicon gate electrode 208.

Then a second layer 272 of a second semiconductor material is formed inthe recesses over the first layer 270 such that it fills the recesses222 and in the embodiment shown in FIG. 12, extends above the surface216 of the epitaxial silicon layer 204. The second semiconductormaterial of the second layer 272 is chosen such that it forms strainedsemiconductor regions 272 in the epitaxial silicon layer 204. In anembodiment according to the second aspect of the invention, the secondsemiconductor material has a lattice structure that is substantially thesame as the lattice structure of the semiconductor layer 204 and has alattice spacing or lattice constant (which is the distance between atomsin the lattice) different to the lattice spacing of the epitaxialsilicon layer 204. The difference in lattice spacing should be largeenough to create strain but not too large to create defects that relievethe strain. To induce a compressive strain for PMOS devices, the latticespacing of the second semiconductor material is required to be greaterthan the lattice spacing of the semiconductor layer 204. Other factorsmay be considered when selecting the second semiconductor material, forexample, it is desirable that the second semiconductor material iscompatible with the semiconductor layer 204 upon which it is depositedat the temperatures used in the semiconductor device fabricationprocess.

The semiconductor material typically includes epitaxially grown siliconand germanium and boron, known as in-situ boron doped SiGe, and isformed by epitaxial deposition on the first layer 270 in the recesses222, for example by a rapid thermal CVD process. Typically, thepercentage of silicon used in the SiGe material is 25-30%. Thepercentage is determined such that the addition of Ge to the Si changesthe lattice constant sufficiently to create strain without creatingdefects that relieve the strain. When too much Ge is present, thelattice constant difference between the semiconductor layer 204 and thesecond semiconductor material becomes so large that defects form torelieve the high stress induced by the lattice constant difference.Thus, the percentage of Si vs Ge is determined by the trade off betweenwanting high stress in the channel but not so high that defects form torelieve the stress and create leakage paths through the junction. Othersemiconductor materials may also be used as the second layer 272, suchas undoped silicon and germanium or other combinations of III-IVsemiconductor materials such as gallium and arsenide.

Instead of having an epitaxial silicon layer 204, the semiconductorlayer 204 may comprises a conventional prime substrate, such as asilicon substrate. In this case, the second semiconductor material maycomprise silicon and germanium (or in-situ doped SiGe) epitaxiallydeposited on the silicon substrate.

The depth of the second layer will depend on the depth of the recess 222and also the depth of the first layer 270. In an embodiment where thefirst layer 270 has a depth 5 nm and the recess has a depth of 70 nm,the second layer 272 will have a depth of 65 nm.

If the gate electrode 208 is etched, then the second semiconductormaterial is also deposited on the first semiconductor material on thepolysilicon gate electrode 208.

The junction between the source and drain regions 270 and the epitaxialsilicon layer 204 is formed a predetermined depth 251 below theinterface between the strained semiconductor regions 272 and theepitaxial silicon layer 204. The predetermined depth 251 is dependent onthe depth of the first layer 270: thus, when the depth of the firstlayer 270 is 5 nm, the predetermined depth is 5 nm.

Other process steps then take place to complete the formation of thePMOS device which are well known in the art, for example, metallizationto form the source, drain and gate electrodes.

The second aspect of the present invention thus forms the source anddrain regions before forming the strained semiconductor regions by meansof using a two step process to deposit the first and second layers ofdifferent materials in the recesses. Thus, as with the first aspect ofthe invention, the second aspect of the present invention ensures thatthe junction between the source and drain regions and the epitaxialsilicon layer is below the interface between the strained semiconductorregion and the epitaxial silicon layer. As discussed above, this has anadvantage of reducing junction leakage which is particularlyadvantageous in low power applications. Moreover, the present inventionenables the depth of the strained semiconductor regions to be increasedwithout requiring deeper implants in order to form the source and drainregions below the strained semiconductor regions. As discussed above,having deeper strained semiconductor regions increases the stress on thechannel which increases charge mobility. Thus, the present inventionenables the structure of a source/drain stressor PMOS device to beoptimised to improve device performance.

Since the second aspect of the present invention forms the source anddrain regions by depositing the first layer comprising semiconductormaterial doped with a dopant rather than by implantation, the source anddrain regions formed in accordance with the second aspect of the presentinvention have a step-shaped cross-section and thus, a junction can beformed that is more abrupt than any junction formed by implantation. Amore abrupt junction is desirable since it increases device performance.

In an embodiment of the second aspect of the invention, the first layercomprising semiconductor material doped with a dopant is deposited onthe epitaxial silicon layer 204. Depositing semiconductor material on anun-implanted surface is easier than depositing semiconductor material ona surface in which dopants have been implanted. Thus, the second aspectof the invention simplifies the manufacturing process compared toforming the strained SiGe regions on an implanted surface and will nothave the complication of defects and amorphization created when usingimplants to form the junction.

The present invention has been described with reference to a PMOSdevice. An NMOS device may be manufactured in a similar manner. In anNMOS device, doping conductivity types would be reversed. For example,phosphorous or arsenic could be used as the dopant for doped regions 112and 150. Furthermore, a tensile stress will be created in the channelusing strained semiconductor regions comprising, for example, siliconand carbon. In the NMOS case, the semiconductor material to be depositedin the recesses is selected such that its lattice structure is the sameas the lattice structure of the semiconductor layer which forms thechannel and its lattice spacing is less than the lattice spacing of thesemiconductor layer which forms the channel.

1. A method of forming a semiconductor device comprising: providing asemiconductor layer; forming a control electrode over a portion of thesemiconductor layer; forming recesses extending into the semiconductorlayer on opposing sides of the control electrode; forming a first layerof a first semiconductor material doped with a dopant in the recesses,in each recess the dopant of the first layer forming a current electroderegion of the semiconductor device the first layer having a depth ofbetween 2 to 5 nm; and forming a second layer of a second semiconductormaterial in the recesses over the first layer, the second semiconductormaterial being different to the first semiconductor material and havinga lattice spacing different to the lattice spacing of the semiconductorlayer such that the second layer forms strained semiconductor regions inthe recesses, wherein a junction between each current electrode regionand the semiconductor layer is formed below an interface between astrained semiconductor region and the semiconductor layer.
 2. The methodas claimed in claim 1, wherein the dopant comprises one of thefollowing: boron, arsenic, phosphorous.
 3. The method as claimed inclaim 1, wherein forming recesses comprises etching the semiconductorlayer to provide the recesses.
 4. The method as claimed in claim 1,wherein the semiconductor device is a planar transistor device and thedoped regions form source and drain regions of the planar transistordevice.
 5. The method of as claimed in claim 1, wherein forming thefirst layer comprises depositing an epitaxially grown in situ dopedsemiconductor material over the semiconductor layer in the recesses. 6.The method of as claimed in claim 1, wherein forming the second layercomprises depositing epitaxially the second semiconductor material. 7.The method as claimed in claim 1, wherein the second semiconductormaterial comprises one of the following materials: epitaxially-grownsilicon germanium, epitaxially-grown in-situ doped silicon germanium,epitaxially-grown silicon carbide, epitaxially-grown in-situ dopedsilicon carbide, epitaxially-grown gallium arsenide, andepitaxially-grown aluminium arsenide.
 8. The method of as claimed inclaim 1, wherein forming the second layer comprises depositingepitaxially the second semiconductor material.
 9. The method as claimedin claim 8, wherein the second semiconductor material comprises one ofthe following materials: epitaxially-grown silicon germanium,epitaxially-grown in-situ doped silicon germanium, epitaxially-grownsilicon carbide, epitaxially-grown in-situ doped silicon carbide,epitaxially-grown gallium arsenide, and epitaxially-grown aluminiumarsenide.
 10. The method as claimed in claim 9, wherein forming recessescomprises etching the semiconductor layer to provide the recesses. 11.The method as claimed in claim 9, wherein the dopant comprises one ofthe following: boron, arsenic, phosphorous.
 12. The method as claimed inclaim 11, wherein forming recesses comprises etching the semiconductorlayer to provide the recesses.
 13. The method as claimed in claim 12,wherein the semiconductor device is a planar transistor device and thedoped regions form source and drain regions of the planar transistordevice.